This invention relates to passive optical components containing chlorine-doped regions, and more particularly, to fiber optic couplers incorporating optical fibers or paths wherein chlorine-doped regions are at least partially responsible for a difference in propagation constants in the optical paths.
Coupling can occur between two closely spaced cores or optical paths in passive optical devices. Such devices exist in the form of fused fibers, overclad fused fibers, planar devices and the like. Fused fiber devices are disclosed in U.S. Pat. Nos. 4,798,436 (Mortimore) and 4,822,126 (Sweeney et al.) and in the publication: O. Parriaux et al., "Wavelength Selective Distributed Coupling Between Single Mode Optical Fibers for Multiplexing", Journal of Optical Communications, Vol. 2, No. 3, pp. 105-109. Overclad fiber optic couplers are disclosed in European published patent application No. 0302745 and in said related U.S. patent application Ser. No. 07/447,808. Examples of planar passive devices can be found in U.S. Pat. Nos. 3,957,341 (Taylor) and 4,765,702 (Dohan et al).
In the discussion of passive devices having regions of different refractive index, the relative refractive index difference .DELTA..sub.a-b between two materials with refractive indices n.sub.a and n.sub.b is defined as EQU .DELTA..sub.a-b =(n.sub.a.sup.2 -n.sub.b.sup.2)/2n.sub.a.sup.2 (1)
For simplicity of expression, .DELTA. is often expressed in per cent, i.e. one hundred times .DELTA..
Couplers exhibiting diverse spectral coupling characteristics such as achromaticity, narrow band wavelength division multiplexing (WDM) and the like, can be formed from optical paths or fibers having different propagation constants. The aforementioned patents and publications teach that two optical paths can be provided with a .DELTA..beta., i.e. a difference in propagation constants, by providing the core and/or cladding portions of those paths with different cross-sectional areas or different refractive index profiles.
Certain of the aforementioned coupling effects can be achieved by employing fibers having slightly different cladding refractive indices. This cladding refractive index difference can be the sole effect employed to obtain a .DELTA..beta. between the fibers, or it can be utilized in combination with one or more other effects that change propagation constant. This index difference can be characterized in terms of .DELTA..sub.clads, the value of which is obtained by substituting the cladding index n.sub.2 of one fiber and the cladding index n'.sub.2 of the other fiber for n.sub.a and n.sub.b, respectively, of equation (1) and solving for .DELTA.. The required value of .DELTA..sub.clads can be obtained by adding a dopant to the cladding of only one of the fibers or by adding different amounts of the same or different dopants to the claddings of the two fibers. Certain devices, such as the achromatic overclad coupler disclosed in said related U S patent application Ser. No. 07/447,808, require a very small value of .DELTA..sub.clads, namely less than 0.03%, to obtain the desired value of .DELTA..beta.. Heretofore, it has been conventional practice to employ silica as the base glass and to employ dopants such as fluorine or B.sub.2 O.sub.3 to lower the refractive index or to employ GeO.sub.2, P.sub.2 O.sub.5 or the like to increase the refractive index of the base glass. When a relatively low value of .DELTA..sub.clads is required to obtain the desired value of .DELTA..beta., it has been difficult to employ dopants such as B.sub.2 O.sub.3, fluorine, GeO.sub.2 and the like since they have a relatively large effect on refractive index. It is therefore difficult to deliver such dopants in the small, precisely controlled amounts that are necessary to change the refractive index of the base glass to an extent sufficient to provide such small values of .DELTA..sub.clads.